1. Field of the Invention
This invention relates to semiconductor processing. More particularly, this invention relates to improvements in the process of depositing refractory metal layers on semiconductor substrates using sequential deposition techniques.
2. Description of the Related Art
The semiconductor industry continues to strive for larger production yields while increasing the uniformity of layers deposited on substrates having increasingly larger surface areas. These same factors in combination with new materials also provide higher integration of circuits per unit area on the substrate. As circuit integration increases, the need for greater uniformity and process control regarding layer characteristics rises. Formation of refractory metal layers in multi-level integrated circuits poses many challenges to process control, particularly with respect to contact formation.
Contacts are formed by depositing conductive interconnect material in an opening on the surface of insulating material disposed between two spaced-apart conductive layers. The aspect ratio of such an opening inhibits deposition of conductive interconnect material that demonstrates satisfactory step coverage and gap-fill, employing traditional interconnect material such as aluminum. In addition, the resistance of aluminum has frustrated attempts to increase the operational frequency of integrated circuits.
Attempts have been made to provide interconnect material with lower electrical resistivity than aluminum. This has led to the substitution of copper for aluminum. Copper suffers from diffusion resulting in the formation of undesirable intermetallic alloys that require the use of barrier materials.
Barrier layers formed from sputtered tantalum (Ta) and reactive sputtered tantalum nitride (TaN) have demonstrated properties suitable for use with copper. Exemplary properties include high conductivity, high thermal stability and resistance to diffusion of foreign atoms. However, sputter deposition of tantalum and/or tantalum nitride films is limited to use for features of relatively large sizes, e.g., >0.3 μm and contacts in vias having small aspect ratios.
A CVD process offers an inherent advantage over a PVD process of better conformability, even in small structures 0.25 μm with high aspect ratios. As a result, CVD deposition of tantalum and tantalum nitride with various metal-organic sources has been employed. Examples of metal-organic sources include tertbutylimidotris(diethylamido) tantalum (TBTDET), pentakis(dimethylamido) tantalum (PDMAT) and pentakis(diethylamido) tantalum (PDEAT).
Attempts have been made to use existing CVD-based tantalum deposition techniques in an atomic layer deposition (ALD) mode. Such attempts, however, suffer drawbacks. For example, formation of tantalum films from tantalum pentachloride (TaCl5) may require as many as three treatment cycles using various radial based chemistries to perform reduction process of the tantalum to form tantalum nitride. Processes using TaCl5 may suffer from chlorine contamination within the tantalum nitride layer.
There is a need, therefore, for tantalum chemistries that may be employed with fewer reduction steps and shorter cycle times.